As concern for protection of the environment increases, regulations for preventing the escape to the atmosphere of volatile hydrocarbon fuel vapors are being promulgated by governmental agencies. One source of these hydrocarbon vapors is the fuel tanks of vehicles using gasoline or other hydrocarbon fuels of high volatility. With present automotive gas tank designs, fuel vapor can escape during the filling of tanks, and usually even after the fuel tank is filled. Furthermore, the person filling the tank with fuel from a gasoline pump can "overfill" the tank which significantly increases the escape of fuel vapor and often produces spit-back of liquid fuel which rapidly releases hydrocarbon vapor into the atmosphere.
The fuel dispensing nozzle of a typical station fuel pump has an automatic shut-off which is actuated by liquid fuel rising sufficiently in the fill tube of the fuel tank to at least momentarily cover and close off a control port immediately adjacent the outlet end of the dispensing nozzle. Closing this control port causes a vacuum actuated diaphragm and valve to automatically shut off the discharge of liquid fuel from the dispensing nozzle. With a typical prior vehicle fuel tank, this automatic shut off occurs when the fuel tank has been fully filled to the fuel level desired to provide a vapor dome in the tank above the liquid fuel having a predetermined desired minimum volume. However, by again manually actuating the fuel dispensing nozzle sufficient additional liquid fuel can be forced into the tank to create an overfilled condition which undesirably decreases or even eliminates the vapor dome in the tank, leaves liquid fuel in the tank fill pipe, produces substantial spit back of liquid fuel and sometimes discharges excess liquid fuel and fuel vapors to the atmosphere through the open tank fill pipe and/or a tank vent.